Detecting Loss of Alignment of Optical Imaging Modules

ABSTRACT

Imaging apparatus includes a housing, with imaging optics mounted in the housing and configured to form an optical image, at a focal plane within the housing, of an object outside the housing. An image sensor, including a matrix of detector elements, is positioned at the focal plane in alignment with the imaging optics and is configured to output an electronic image signal in response to optical radiation that is incident on the detector elements. At least one emitter is fixed within the housing and is configured to emit a test beam toward one or more reflective surfaces within the housing, which reflect the test beam toward the image sensor. A processor is configured to process the electronic image signal output by the image sensor in response to the reflected test beam so as to detect a change in the alignment of the image sensor with the imaging optics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 62/730,023, filed Sep. 12, 2018; U.S. Provisional PatentApplication 62/818,123, filed Mar. 14, 2019; and U.S. Provisional PatentApplication 62/833,718, filed Apr. 14, 2019. All of these relatedapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to opto-electronic devices, andparticularly to beam-forming optics for optical emitters andapplications of such emitters.

BACKGROUND

Light emitters, such as vertical-cavity surface-emitting lasers(VCSELs), are commonly integrated with a small lens (referred to as amicrolens) that directs and collimates the emitted beam. An array ofsuch microlenses may be fabricated integrally over a semiconductorsubstrate on which an array of emitters is formed, with the microlensesin alignment with the emitters.

Compact optical imaging modules are ubiquitous in portable digitaldevices, such as mobile phones and tablet computers. A typical modulecomprises imaging optics, comprising one or more lenses, and an imagesensor located in the image plane of the optics. Even when the imagingoptics and image sensor have been carefully aligned at the time ofmanufacture, the alignment may shift during the lifetime of the modulein the field, for example due to mechanical shocks.

Methods for detecting and correcting for alignment shift are known inthe art. For example, U.S. Patent Application Publication 2018/0041755describes a method in which a scene is imaged using an imaging system,which includes an array of radiation sensing elements. The arrayincludes first sensing elements with symmetrical angular responses andsecond sensing elements with asymmetrical angular responses,interspersed among the first sensing elements, with optics configured tofocus radiation from the scene onto the array. The method includesprocessing first signals output by the first sensing elements in orderto identify one or more areas of uniform irradiance on the array, andprocessing second signals output by the second sensing elements that arelocated in the identified areas, in order to detect a misalignment ofthe optics with the array.

SUMMARY

Some embodiments of the present invention that are described hereinbelowprovide integrated emitter devices and methods for their production anduse. Other embodiments provide methods and apparatus for detecting lensmisalignment in an optical imaging module, including apparatus usingintegrated emitter devices.

There is therefore provided, in accordance with an embodiment of theinvention, imaging apparatus, including a housing and imaging opticsmounted in the housing and configured to form an optical image, at afocal plane within the housing, of an object outside the housing. Animage sensor, including a matrix of detector elements, is positioned atthe focal plane in alignment with the imaging optics and is configuredto output an electronic image signal in response to optical radiationthat is incident on the detector elements. At least one emitter is fixedwithin the housing and is configured to emit a test beam toward one ormore reflective surfaces within the housing, which reflect the test beamtoward the image sensor. A processor is configured to process theelectronic image signal output by the image sensor in response to thereflected test beam so as to detect a change in the alignment of theimage sensor with the imaging optics.

In some embodiments, the processor is configured to initiate acorrective action upon detecting that a magnitude of the change isgreater than a predefined limit. In one such embodiment, the detectedchange includes a shift of the optical image on the image sensor, andthe corrective action includes processing the electronic image signal soas to compensate for the shift.

In a disclosed embodiment, the at least one emitter is fixed at alocation adjacent to the image sensor.

In some embodiments, at least one of the one or more reflective surfacesis an interior surface of the housing. In a disclosed embodiment, theinterior surface is configured as an elliptical mirror, which focusesthe reflected test beam to form a predefined geometrical figure on theimage sensor, wherein the processor is configured to detect the changein the alignment responsively to movement of geometrical figure on theimage sensor.

Additionally or alternatively, the imaging optics include one or morelenses having refractive surfaces, and the one or more reflectivesurfaces include one or more of the refractive surfaces of at least oneof the lenses. In a disclosed embodiment, the imaging optics areconfigured to form the optical image of the object within a predefinedspectral range, and the test beam is emitted at a wavelength outside thepredefined spectral range, and at least one of the refractive surfacesincludes a coating configured to pass the optical radiation within thepredefined spectral range while reflecting the test beam. Alternativelyor additionally, a pattern is formed on the at least one of the lensesin an area on which the test beam is incident, wherein the patterncauses the reflected test beam to form a predefined geometrical figureon the image sensor, and the processor is configured to detect thechange in the alignment responsively to changes in the geometricalfigure on the image sensor.

Further additionally or alternatively, the at least one emitter includesa radiation source, which is configured to generate the test beam, and alens, which is configured to collimate and direct the test beam towardthe one or more reflective surfaces. In one embodiment, the lensincludes a microlens, such as a micro-prism-lens, which is decenteredrelative to the radiation source in order to direct the test beam at adesired propagation angle toward the one or more reflective surfaces.

In a disclosed embodiment, the at least one emitter includes a pluralityof emitters disposed at different, respective locations within thehousing.

There is also provided, in accordance with an embodiment of theinvention, a method for imaging, which includes mounting imaging opticsin a housing so as to form an optical image of an object outside thehousing on an image sensor, including a matrix of detector elements, inalignment with the imaging optics at a focal plane of the imaging opticswithin the housing. At least one emitter is fixed within the housing soas to emit a test beam toward one or more reflective surfaces within thehousing, which reflect the test beam toward the image sensor. Anelectronic image signal that is output by the image sensor in responseto the reflected test beam is processed so as to detect a change in thealignment of the image sensor with the imaging optics.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an opto-electronic device, inaccordance with an embodiment of the invention;

FIG. 2 is a schematic sectional view of an opto-electronic device, inaccordance with another embodiment of the invention;

FIG. 3 is a schematic sectional view showing a detail of theopto-electronic device of FIG. 2, in accordance with an embodiment ofthe invention;

FIGS. 4-8 are schematic sectional views of opto-electronic devices, inaccordance with further embodiments of the invention;

FIGS. 9A-C are schematic sectional diagrams illustrating a process forfabricating asymmetrical structures on a substrate, in accordance withan embodiment of an invention;

FIGS. 10A, B, C, D, and E are schematic sectional diagrams illustratinga process for producing microlenses on a semiconductor substrate, inaccordance with an embodiment of the invention;

FIG. 11 is a flowchart that schematically illustrates a fabricationprocess of a micro-prism-lens, in accordance with an embodiment of theinvention

FIG. 12 is a schematic sectional view of an optical imaging module withan emitter for detection of changes in alignment, in accordance with anembodiment of the invention;

FIG. 13 is a schematic sectional view of the optical imaging module ofFIG. 12, showing the effect of a change in alignment; and

FIGS. 14-16 are schematic sectional views of optical imaging moduleswith emitters for detection of changes in alignment, in accordance withother embodiments of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Emitters with Micro-Prism-Lenses

Portable electronic devices, such as cellular phones or tablets,commonly employ one or more integral light sources. These light sourcesmay, for example, provide illumination for a scene recorded by a cameraintegrated into the device. They commonly comprise a vertically emittinglight source, such as a VCSEL, and are constrained to a very smallsurface area. Moreover, the light sources in these applications may berequired to tilt the beam of light emitted by a VCSEL with respect toits substrate and to collimate it.

A VCSEL without any additional collimating optics emits a highlydiverging beam in a direction perpendicular to the substrate. Adding amicrolens for beam collimation and tilt is not an optimal solution:Inside the microlens, the VCSEL beam has a low divergence due to thehigh refractive index of the microlens, leading to a small beam diameterbefore the collimating surface, which, in turn, leads to an inherentdivergence due to diffraction. Using the microlens to achieve beam tiltby offsetting the optical axis of the lens from the VCSEL leads to afurther reduction of the beam diameter in the direction of the tilt as afunction of the cosine of the tilt angle.

Some embodiments of the present invention that are described hereinaddress the above limitations so as to provide a compact opticalelement, which focuses and deflects the beam emitted by a solid-stateemitter, such as a VCSEL, so that the beam exits the optical element ata high tilt angle and with large diameter, relative to solutions thatare known in the art. In the disclosed embodiments, an emitter is formedon a semiconductor substrate with a planar surface, and a reflectivelayer (either using a reflective coating or total internal reflection)is formed on the planar surface adjacent to the emitter. Amicro-prism-lens is then formed over the emitter.

The emitter emits a beam of light in a direction away from the planarsurface, for example in a vertical direction relative to the surface.The surface of the micro-prism-lens has a first segment positioned abovethe emitter so as to reflect the emitted beam towards the reflectivelayer, which further reflects the beam towards a second segment of thesurface of the micro-prism-lens. The second segment is formed so as tocollimate and transmit the beam out from the micro-prism-lens. The term“collimate” is used in the context of the present description and claimsto mean that the divergence of the emitted beam is substantiallyreduced, typically by at least 50% in angular terms, even if the emittedbeam is not fully parallelized by the second segment.

A large-diameter collimated and tilted beam is achieved by twoproperties of the disclosed micro-prism-lens:

-   -   1. The two internal reflections within the micro-prism-lens        provide an increased propagation length before the collimating        surface provided by the second segment, thus yielding an        increased diameter of the collimated beam; and    -   2. The reflection of the beam from the first segment of the        surface of the micro-prism-lens imposes the desired tilt on the        beam without reducing its diameter.

Although the embodiments described below relate, for the sake ofsimplicity, to a single VCSEL emitter and micro-prism-lens, theprinciples of the present invention can readily be applied to emittersof other types, as well as to provide integrated arrays ofmicro-prism-lenses over arrays of VCSELs and other emitters.

Embodiments with Two Internal Reflections

FIG. 1 is a schematic sectional view of an opto-electronic device 21, inaccordance with an embodiment of the invention.

Opto-electronic device 21 comprises a planar semiconductor substrate 10,for example comprising GaAs, on which a VCSEL 12 is formed by processesof semiconductor device fabrication that are known in the art. VCSEL 12emits a beam of light 14 in a vertical direction relative to substrate10, for example at a wavelength between 650 nm and 1300 nm. (The terms“optical radiation” and “light” as used in the present description andin the claims refer generally to any and all of visible, infrared, andultraviolet radiation.) A reflective layer 16 is formed on substrate 10.In the present embodiment, reflective layer 16 comprises a metal, suchas gold, which also serves as an electrode for VCSEL 12, but of extendedsize so as to function also as a reflector. In alternative embodiments,other metals, such as aluminum, may be used instead of gold. In furtheralternative embodiments, emitters such as edge-emitting lasers may beused, wherein either the emitter is oriented so that its emitting faceis parallel to substrate 10, or a micro-mirror is used to direct theemitted beam in a direction away from the substrate, in a verticaldirection or possibly angled relative to the vertical. Alternatively,reflective layer 16 may be separate and independent of the electrodesused to drive the emitter.

A micro-prism-lens 18 is formed over VCSEL 12. Micro-prism-lens 18comprises a material that is transparent at the wavelength of beam 14,such as GaAs, fused silica, SiO₂, epoxy, polymers, or glass. The heightof micro-prism-lens 18 may vary from tens of microns, such as 20 or 30microns, to 2 mm, and width from 60 microns to 6 mm. Micro-prism-lens 18can be fabricated utilizing methods described below with reference toFIGS. 9A-C, 10A-E, and 11. Reflective coatings and/or anti-reflectivecoatings can then be deposited on the outer surface of micro-prism-lens,using thin film deposition and patterning techniques that are known inthe art.

A first segment 20 of an outer surface 22 of micro-prism-lens 18 ispositioned to receive and reflect beam 14 internally into a beam 24.First segment 20 is either flat or concave (as viewed from outside), andis tilted so as to impose a deviation angle φ₁ between beams 14 and 24.Beam 24 is reflected by gold layer 16 into a beam 26, which impinges ona second segment 28 of surface 20. Second segment 28 has a radius ofcurvature selected so as to collimate and transmit beam 24 into a beam30. As noted earlier, the term “collimate” is used in the broader senseof reducing the divergence of a beam, so that beam 30 may be diverging(but less than it would be without micro-prism-lens 18), parallel, oreven converging.

The two internal reflections within micro-prism-lens 18, from firstsegment 20 and from gold layer 16, increase the propagation length fromVCSEL 12 to second segment 28, thus ensuring a sufficient diameter fortransmitted beam 30. The deviation angle φ is responsible for imposing adesired tilt onto beam 24, which tilt then propagates through thereflection from gold layer 16 to beam 26 and subsequently to beam 30. Bya suitable choice of the design parameters of micro-prism-lens 18, thedeviation angle φ₁ may exceed 55 degrees, and the diameter of beam 30 atsecond segment 28 may exceed 70 microns. Details of an optical designare shown in FIG. 3, and described below.

Due to the use of a high-index material for micro-prism-lens 18, such asGaAs (n=2.7), the reflection from first segment 20 takes place as atotal internal reflection (TIR) as long as the angle of incidence ofbeam 14 on the first segment is larger than approximately 20 degrees.Additionally or alternatively, surface 22 may be coated by ananti-reflective (AR) coating 32 in order to reduce reflection losses oftransmitted beam 30.

In an alternative embodiment, the part of gold layer 16 that reflectsbeam 24 into beam 26 may be replaced by a reflective dielectric layer,which reflects beam 24 either by TIR or by virtue of a multilayerconstruction designed for high reflectivity.

The shape of first segment 20 may be determined based on considerationsof both manufacturability and functionality: A planar shape may beeasier to manufacture and less sensitive to tolerances, whereas aconcave shape increases the divergence of beam 24, which then increasesthe diameter of beam 30.

FIG. 2 is a schematic sectional view of an opto-electronic device 23, inaccordance with another embodiment of the invention.

Opto-electronic device 23 is similar to device 21, and the same labelsin FIG. 2 are used for items similar to those in FIG. 1. Opto-electronicdevice 23 comprises a micro-prism-lens 40, which is similar tomicro-prism-lens 18 of device 21, with a flat first segment 42, andfunctions in a similar fashion to device 21. When device 23 isincorporated in an illumination array, an array of micro-prism-lenses 40of this sort can be formed in alignment over a corresponding array ofVCSELs 12.

FIG. 3 is a schematic sectional view of a detail 44 of opto-electronicdevice 23, showing an optical design, in accordance with an embodimentof the invention. For the sake of clarity, anti-reflective coating 32has been omitted from the figure, but may be included in thisembodiment, and second segment 28 is illustrated as a flat surface (with“R” to indicate its radius of curvature). FIG. 3 shows an optical designthat yields collimated beam 30 with a diameter of 80 microns and a tiltangle φ₂ 54 degrees with respect to a vertical direction. The thicknessH of micro-prism-lens 40 is 95 microns, the horizontal center-to-centerseparation L between first segment 42 and second segment 28 is 700microns, and the radius of curvature R of the second segment is 705microns, with the second segment convex toward the outside of themicro-prism-lens. Alternatively, by scaling all the dimensions by afactor of X, the same tilt angle φ₂ of 54 degrees for collimated beam 30is achieved, with beam diameter D scaled down to D/X=80/X microns.

FIG. 4 is a schematic sectional view of an opto-electronic device 25, inaccordance with yet another embodiment of the invention.

Opto-electronic device 25 is similar to device 21, and the same labelsare used in FIG. 4 for items similar to those in FIG. 1. Opto-electronicdevice 25 comprises a micro-prism-lens 50, which is similar tomicro-prism-lens 18 of device 21, except for a first segment 52, whichis formed as a concave indentation in the flat outer surface ofmicro-prism-lens 50. This concave shape is advantageous in increasingthe divergence of beam 24, relative to the preceding embodiments.

Alternative Embodiments

FIG. 5 is a schematic sectional view of an opto-electronic device 55, inaccordance with another embodiment of the invention. In this embodimentand the embodiments that follow, the beam from VCSEL 12 is reflectedinternally four or more times within the micro-prism-lens, thusincreasing the optical path length and hence increasing the diameter ofthe beam that is projected out of the device.

Opto-electronic device 55 comprises the following items that are similarto those of device 21, and are labeled with the same labels: planarsemiconductor substrate 10, VCSEL 12, and gold layer 16. As in device21, VCSEL 12 emits beam of light 14 in a vertical direction relative tosubstrate 10. A micro-prism-lens 60 is formed over VCSEL 12.Micro-prism-lens 60 comprises, as in device 21, a material that istransparent at the wavelength of beam 14.

A first segment 62 of a surface 64 of micro-prism-lens 60 comprises afirst and a second sub-segment 62 a and 62 b, respectively, wherein bothsub-segments are planar, but are not co-planar. A second segment 66 ofsurface 64 is similar to second segment 28 of device 21. Second segment66 is coated with an AR coating 68, whereas first segment 62 is coatedwith a reflective coating 70, comprising aluminum, for example.

First sub-segment 62 a receives beam 14 and reflects it into a beam 72with an angle of deviation of φ₄, after which beam 72 is reflected bygold layer 16 into a beam 74. Beam 74 impinges on second sub-segment 62b, and is reflected into a beam 76 with an angle of deviation of φ₅.Beam 76 is reflected by gold layer 16 into a beam 78, which issubsequently collimated and transmitted by second segment 66 into a beam80.

The two angles of deviation, φ₄ and φ₅, can be controlled by adjustingthe tilt angles of sub-segments 62 a and 62 b, thus enabling control ofboth the tilt angle and the diameter of collimated beam 80. Inalternative embodiments (not shown in the figures), first segment 62 maycomprise more than two sub-segments, such as, for example, three, four,or even five subsegments. Additionally or alternatively, one or more ofthe surfaces of the sub-segments of first segment 62 may be non-planar(i.e., concave or convex) for further control of the beam size. Forexample, a concave shape may be used to increase the diameter ofcollimated beam 80, whereas a convex shape may be used for eitherincreasing or decreasing the beam diameter.

FIG. 6 is a schematic sectional view of an opto-electronic device 85, inaccordance with yet another embodiment of the invention.

Opto-electronic device 85 comprises the following items that are similarto those of device 21, and are labeled with the same labels: planarsemiconductor substrate 10, VCSEL 12, and gold layer 16. Amicro-prism-lens 90 is formed over VCSEL 12 and comprises, as in device21, a material that is transparent at the wavelength of beam 14.

A first segment 92 of a surface 94 of micro-prism-lens 90 is planar, anddimensioned so as to intercept at least two reflections from gold layer16, as will be detailed below. A second segment 96 of surface 94 issimilar to second segment 28 of device 21. Similarly to opto-electronicdevice 55, second segment 96 is coated with an AR coating 98, whereasfirst segment 92 is coated with a reflective coating 100.

First segment 92 receives beam 14 and reflects it into a beam 102 withan angle of deviation of φ₆, after which beam 102 is reflected by goldlayer 16 into a beam 104. Beam 104 impinges back on first segment 92,and is again reflected to gold layer 16, now as a beam 106 with an angleof deviation of φ₇. Beam 106 is reflected by gold layer 106 into a beam108, which is subsequently collimated and transmitted by second segment96 into a collimated beam 110. A central ray 112 of beam 108 and acentral ray 114 of beam 110 will be used for comparison in FIG. 7.

With an appropriate choice of width of first segment 92 and its tiltangle with respect to gold layer 16, the first segment may interceptmore than two reflections from the gold layer, for example, three, four,or five reflections. Moreover, the beams originating from differentreflections from gold layer 16 and impinging on first segment 92 areallowed to overlap on the first segment.

FIG. 7 is a schematic sectional view of an opto-electronic device 95, inaccordance with a further embodiment of the invention.

Opto-electronic device 95 is identical to opto-electronic device 85 ofFIG. 6, except for a second segment 122 of a micro-prism-lens 120. Forall the other items of opto-electronic device 95 the same labels areused as for device 85, and from VCSEL 12 until second segment 122 indevice 95, the beams of light follow paths identical to those in device85.

In device 95, second segment 122 transmits and collimates beam 108 intoa beam 124 in such a way that the axis of beam 124 (as represented by acentral ray 126 of the beam) is deviated from the axis of beam 108 (asrepresented by central ray 112) by an angle α with respect to a normalto the second segment, whereas in device 85 central ray 108 continues ina straight line relative to central ray 114. Thus, by choosing the tiltof second segment 122 with respect to central ray 112, the direction ofcollimated beam 124, and specifically its central ray 126, may be chosento be either collinear with central ray 112 or at an elevated or loweredangle with respect to central ray 112. The design parameters ofmicro-prism-lens 120 may be chosen to yield a desired tilt angle ofcentral ray 126 with respect to substrate 10. For example, central ray126 may be perpendicular to substrate 10, as shown by a dotted line 128.Due to the increased diameter of beam 108 at second segment 122, aspreviously described, a reduction of the diameter of beam 124 due to thenon-normal tilt angle (α≠0) may be more easily tolerated.

FIG. 8 is a schematic sectional view of an opto-electronic device 135,in accordance with another embodiment of the invention.

Opto-electronic device 135 is similar to section 44 of device 23 (FIGS.2, 3), and the same labels are used in FIG. 8 for items similar to thosein FIGS. 1 and 2. Opto-electronic device 135 comprises amicro-prism-lens 140, which is similar to micro-prism-lens 40 of device23, except for a second segment 142, which is formed either as aFresnel-lens or as another type of diffractive optical element (DOE).This embodiment is suitable for designs wherein beam 26 impinges onsecond segment 142 at a low angle of incidence, such as 10° or less.Additionally or alternatively, first segment 42 may be implemented usinga Fresnel-lens or another type of DOE.

FIGS. 9A-C are schematic sectional diagrams illustrating a process forfabricating asymmetrical structures 160 on a substrate 162, inaccordance with an embodiment of an invention. Asymmetrical structures160 of this sort can be used for the purpose of fabricatingmicro-prism-lenses 18 (FIG. 1), 40 (FIG. 2), 50 (FIG. 4), (FIG. 5), 90(FIG. 6), 120 (FIG. 7), and 140 (FIG. 8), as will be further describedwith reference to FIGS. 10A-D.

Asymmetric structures 160 can be fabricated, for example, using thefabrication process described by Gimkiewicz et al., in “Fabrication ofmicroprisms for planar optical interconnections by use of analoggray-scale lithography with high-energy-beam-sensitive glass,” AppliedOptics, Vol. 38, pp. 2986-2990 (1990), which is incorporated herein byreference. The fabrication process utilizes a HEBS(high-energy-beam-sensitive) glass mask 164, wherein a gray-levelpattern 166 has been exposed by electron-beam lithography, with furtherdetails given in the above-referenced publication by Gimkiewicz et al.

FIG. 9A shows an exposure step of the fabrication process, whereinsubstrate 162, coated by a photoresist layer 168, is exposed throughHEBS mask 164 by ultraviolet (UV) light 170.

FIG. 9B shows the result of a development step, wherein exposedphotoresist layer 168 has been developed into asymmetrical photoresiststructures 172.

FIG. 9C shows the result of an etch step, wherein the shapes ofasymmetrical photoresist structures 172 have been, with the use ofreactive-ion etching (RIE), transferred into substrate 162 to formasymmetrical structures 160.

Although asymmetrical structures 160 have a simple prismatic form, morecomplicated forms comprising, for example, curved surfaces, may beproduced by generating suitable gray-level patterns in HEBS mask 164.Such structures, comprising microlenses 174 as an example, areillustrated in FIG. 10A, below. Substrate 162 may be a substrate that isetchable by RIE and has the required mechanical properties, such asfused silica.

FIGS. 10A-D are schematic sectional diagrams illustrating a process fortransferring microlenses 174 on substrate 162 into a GaAs substrate 176by the Confined Etchant Layer Technique (CELT), in accordance with anembodiment of the invention. The transfer process utilizes a CELT,described by Zhan et al. in “Confined Etchant Layer Technique (CELT) forMicromanufacture,” Proc. 6th IEEE International Conference on Nano/MicroEngineered and Molecular Systems, pp. 863-867 (2011), which isincorporated herein by reference.

FIG. 10A shows a replication of microlenses 174 on substrate 162, whichserves as a mold, into a layer of a polymer, such as PMMA 178. Althoughthe embodiments of FIGS. 10A-D illustrate the transfer of simplemicrolenses 174, more complex structures may be fabricated, as detailedin FIGS. 9A-C, above. The replication process comprises spinning liquidPMMA over microlenses 174, drying the liquid PMMA into solid PMMA 178,and separating the PMMA from the microlenses, so that the curved shapesof the microlenses are replicated in the PMMA.

FIG. 10B shows a start of an etching process for transferring thefeatures on the layer of PMMA 178 into GaAs substrate 176. A surface 180of PMMA 178 has been coated by thin layers of titanium (Ti) and platinum(Pt). It has been brought into close proximity to GaAs substrate 176(for example, within a hundred microns or tens of microns), withmicrolenses 174 facing the GaAs substrate. A space 182 between PMMA 178and GaAs 176 has been filled with a mixture of an etchant and ascavenger, for example with bromide and cystine respectively used asetchant and scavenger. The etchant has been chosen to etch GaAs, but notthe layer of inert Pt. The scavenger is used to control the process, asdetailed by Zhan et al., cited above.

FIGS. 10C, 10D and 10E show the progress of the transfer process, withspace 182 continuously diminishing, until in FIG. 10E, PMMA 178 and GaAs176 have come into contact, and the transfer process has been completed.

Although PMMA is used as a mold in the embodiments described above,other materials, such as silicon or a platinum-iridium (Pt—Ir), alloymay be used as molds when suitably patterned.

FIG. 11 is a flowchart that schematically illustrates the fabricationprocess of a micro-prism-lens, in accordance with an embodiment of theinvention. This flowchart illustrates schematically the processes shownin FIGS. 9A-C and 10A-E for fabricating micro-prism-lenses 18 (FIG. 1),40 (FIG. 2), 50 (FIG. 4), 60 (FIG. 5), 90 (FIG. 6), 120 (FIG. 7), and140 (FIG. 8). The additional fabrication steps of opto-electronicdevices 21, 23, 25, 55, 85, 95, and 135, such as fabrication of a VCSELand depositing of anti-reflective and reflective coatings, have not beenincluded in the flowchart for the sake of simplicity, and theirimplementation will be apparent to those skilled in the art. In thedescription below, reference is also made to FIGS. 9A-C and 10A-E.

The fabrication starts in a start step 184. In parallel (but notnecessarily concurrently), HEBS mask 164 is fabricated in a maskfabrication step 186, and substrate 162 is coated by photoresist 168 ina photoresist coating step 188. In an exposure step 190, photoresist 168is exposed through HEBS mask 164, as shown in FIG. 9A. In a developmentstep 192, exposed photoresist 168 is developed to produce structures172, as shown in FIG. 9B. In an RIE step 194, structures 160 are etchedinto substrate 162, as shown in FIG. 9C.

In a transfer step 196, structures 174 on substrate 162 are transferredinto PMMA 178, as shown in FIG. 10A. In a PMMA coat step 198, thinlayers of Ti and Pt are deposited on PMMA 178. In an etch cell assemblystep 200, PMMA 178 is brought close to GaAs substrate 176, and space 182is filled with etchant and scavenger, as shown in FIG. 10B. In an etchstep 202, structures on PMMA 178 are transferred into GaAs 176, as shownin FIGS. 10C-E. In a separation step 204, PMMA 178 and GaAs 176 areseparated from each other. The process ends in an end step 206.

Features of the embodiments shown in FIGS. 1-11 may be combined inadditional ways, as will be apparent to those skilled in the art afterreading the present description.

Detecting Loss of Alignment in Optical Imaging Modules

The position and tilt of the imaging optics relative to the image sensorin an optical imaging module can play a critical role in the performanceof the module. In particular, when the imaging module is used inmeasurement applications, changes in alignment can lead to inaccuratemeasurements. For example, many depth mapping systems project a patternof structured light onto a scene, and then analyze an image of thepattern that is captured by an imaging module in order to compute depthcoordinates of objects in the scene by triangulation. A change of theposition or tilt of the imaging optics relative to the image sensor canresult in a significant error in the estimation of the depth.

Some embodiments of the present invention that are described hereinaddress this problem by periodically sensing the alignment of the imagesensor with the imaging optics, and initiating corrective action when asignificant shift in alignment is detected. The disclosed embodimentsmake use of one or more dedicated emitters, which can be built into theoptical imaging module and direct beams of radiation toward one or moreof the internal surfaces in the module. These surfaces can include, forexample, refractive entrance and exit faces of lenses in the imagingoptics, as well as interior surfaces of the lens housing. The emitter oremitters are arranged so that the radiation reflected from the internalsurface or surfaces is incident on the image sensor. Changes in thepattern of this reflected radiation on the image sensor give anindication of changes in the alignment of the optics with the imagesensor.

The disclosed embodiments provide imaging apparatus, such as an opticalimaging module, comprising imaging optics, which are mounted in ahousing and form an optical image of an object outside the housing. Animage sensor, comprising a matrix of detector elements, is positionedand aligned at the focal plane of the imaging optics within the housing.In addition to these standard imaging module components, at least oneemitter is fixed within the housing and emits a test beam toward one ormore reflective surfaces within the housing. These reflective surfaces,which reflect the test beam toward the image sensor, may comprise, forexample, one or more of the refractive surfaces of the lenses in theimaging optics and/or an interior surface of the housing. The test beammay be a directional beam, which is aimed in the desired direction bythe sort of micro-prism-lens that is described above.

The image sensor outputs electronic image signals in response to theoptical radiation that is incident on the detector elements, includingsignals in response to the reflected test beam. (The emitter may beactuated to emit the test beam only for short intervals, for examplewhen the imaging module is not in use.) If the alignment of the imagesensor with the imaging optics changes, for example due to a mechanicalshock, the electronic image signal output by the image sensor due to thereflected test beam will change, as well. A processor receives andprocesses this electronic image signal in order to detect such changes,and will initiate corrective action when the magnitude of the change isgreater than a predefined limit.

For example, a change in the electronic image signal due to the testbeam may indicate that the optical image formed by the imaging optics onthe image sensor has shifted as the result of a shift in the opticalcenter of the imaging optics. In this case, the processor may compensatefor the shift, possibly by applying a counter-shift of the appropriatedirection and magnitude to the electronic images output by the imagesensor.

Embodiments of the present invention are thus useful in enhancing theaccuracy of measurement applications, such as depth sensing, that useimaging modules. Such embodiments can be used to correct certain errorsthat may result from changes in alignment of the imaging module, and/orto issue an alert when an error is not readily correctable, andmeasurements are likely to be incorrect. The disclosed embodimentsrequire only minimal hardware additions to imaging module designs, whiletaking advantage of the existing image sensor and associated imageprocessing capabilities.

For the sake of simplicity in the embodiments that are shown in thefigures and described below, only a single emitter is used in generatinga test beam. In alternative embodiments, however, multiple emitters aredisposed at different, respective locations within the housing, so thateach directs a respective beam toward the reflective surfaces of theoptics at a different respective angle. The emitters may be operated insequence, thus generating a sequence of different electronic imagesignals from the image sensor. The processor can analyze these signalsin order to extract a comprehensive picture of any changes in alignmentthat may have occurred. For example, two emitters may be positioned 90°apart around the optical axis and thus be used in detecting opticalshifts along two orthogonal axes. Alternatively or additionally, twoemitters may be positioned in close proximity to one another in order toincrease the measurement precision.

Reference is now made to FIGS. 12 and 13, which are schematic sectionalviews of an optical imaging module 220, in accordance with an embodimentof the invention. FIG. 12 shows the components of module 220 in aninitial, baseline position, for example following factory calibration ofthe module, while FIG. 13 shows the effect of a shift in alignment.Although this figure relates only to alignment shift due to elementdecentering, the principles of the present invention may similarly beapplied in detecting other sorts of misalignment, due to element tiltand deformation, for example.

Module 220 comprises imaging optics in the form of lenses 222, labeledA, B, C and D for convenience, which are mounted in a housing 224.Housing 224 may comprise a lens barrel or any other suitable sort oflens mount. Lenses 22 form optical images, at a focal plane within thehousing, of objects outside the housing. An image sensor 226 ispositioned at the focal plane in alignment with lenses 222. Image sensor226 typically comprises a matrix of detector elements, as is known inthe art, and outputs an electronic image signal in response to opticalradiation that is incident on the detector elements. (The term “opticalradiation,” as used in the present description and in the claims, refersto electromagnetic radiation in any of the visible, infrared andultraviolet ranges and may be used interchangeably with the term“light.”)

An emitter 228 is fixed within housing 224 and emits a test beam 230toward one or more reflective surfaces within the housing, which reflectthe test beam toward image sensor 226. Emitter 228 typically comprises aminiature radiation source, such as a semiconductor laser (includingboth edge-emitting and vertical-cavity surface-emitting laser (VCSEL)types), or light-emitted diode (LED), and may be conveniently fixed at alocation adjacent to image sensor 226, for example on the same circuitsubstrate. In the present embodiment, emitter 228 emits test beam 230toward a reflective interior surface 232 of housing 224.

A processor 234 receives and processes the electronic image signaloutput by image sensor 226 in response to the reflected test beam, andthus detects possible changes in the alignment of the image sensor withthe imaging optics. As noted earlier, only short actuation times ofemitter 228 are needed for this purpose, so that the effect on thenormal operation of module 220 is minimal. Processor 234 typicallycomprises a programmable microprocessor or microcontroller, which isprogrammed in software or firmware to carry out the functions that aredescribed herein, and which has suitable input and output interfaces forreceiving the electronic image signals from image sensor 226 andoutputting alerts and control signals as appropriate. Typically(although not necessarily), processor 234 also performs other processingand control functions in module 220, such as processing images that areformed on image sensor 226 by lenses 222, for example for purposes ofdepth mapping. Alternatively or additionally, processor 234 compriseshardware logic circuits, which may be hard-wired or programmable.

In the example shown in FIG. 12, reflective surface 232 is shaped as aconcave curved mirror, such as an elliptical mirror, which reflects andfocuses test beam 230 so as to form a predefined geometrical figure onimage sensor 226. The reflective surface is polished but does notgenerally require an actual mirror coating. Processor 234 is configuredto detect changes in the alignment responsively to movement ofgeometrical figure on the image sensor.

A change of this sort is shown in FIG. 13, in which housing 224 andlenses 222 have shifted relative to image sensor 226, as indicated by anarrow 236, with the result that the optical center of the imaging opticshas shifted relative to the image sensor. In this case, processor 234will detect that the geometrical figure formed on image sensor 226 byreflection of test beam 230 from surface 232 has shifted, as well.Alternatively, movements along other axes can be detected using thistechnique, including movements along the direction of the optical axisof lenses 222, rather than perpendicular to the optical axis as shown inFIG. 22.

Surface 232 may advantageously be shaped as a cylindrical-ellipticalmirror, so that the geometrical figure formed on image sensor 226 is acurved line. Processor 234 can detect movement of this line in thedirection of arrow 236 (following a mechanical shock to module 220, forexample) with high resolution, particularly using sub-pixel detectionalgorithms, as are known in the art. The configuration of the ellipticalmirror that is shown in FIGS. 12 and 13 creates an amplification in themovement of the line formed on image sensor 226, so that if housing 224moves by a distance x, the line on the image sensor can move by 25x ormore. Combining this movement amplification with sub-pixel detectionenables processor 234 to detect relative shifts between the image sensorand imaging optics that are as small as 0.1 pixels, or possibly less.

FIG. 14 is a schematic sectional view of an optical imaging module 240,in accordance with another embodiment of the invention. Elements in thisfigure, as well as in the figures that follow, that are identical orclosely similar to their counterparts in optical imaging module 220 aremarked with the same indicator numbers as in FIG. 12. Processor 234 isomitted from these figures for the sake of simplicity.

In module 240, an emitter 242 emits a test beam 244 with high angulardivergence toward both surface 232 of housing 224 and toward therefractive surfaces of lenses 222. This arrangement gives rise tomultiple reflections of beam 244 from the various lens, which areincident on image sensor 226. For the sake of simplicity, however, FIG.23 shows only reflected beams 248 and 250, which are reflected from andfocused by respective concave surfaces 249 and 252 of lenses D and B,along with a reflected beam 246 from surface 232.

Processor 234 processes the electronic image signals due to reflectedbeams 246, 248 and 250 (and possibly beams reflected from other lenssurfaces, whether concave, convex or any other shape) in order to detectchanges in the reflected beams that may be indicative of changes inalignment. The changes in this case may be either in the positions ofindividual lenses 222 or of housing 224 as a whole relative to imagesensor 226. A numerical ray-trace simulation can be used to analyze thepattern of reflected beams that appears on image sensor 226 and theinfluence of movement of various elements of module 240 on the pattern,including the effect of lens refraction on the test beam. Processor 234can use the simulation results in matching and analyzing the patternsthat are formed on the image sensor by the reflected beams, and can thustranslate particular pattern changes into the movements that caused thechanges. Additionally or alternatively, machine learning algorithms canbe used in associating changes in the patterns on the image sensor withmovements of elements of module 240.

In many imaging applications, lenses 222 are configured to form opticalimages of objects in a scene within a certain predefined spectraloperating range. Emitter 242 may be chosen to emit test beam 244 at awavelength outside this spectral range. Frequently, some or all of therefractive surfaces of lenses 222, such as surfaces 249 and 252, arecoated to pass the optical radiation within the predefined spectraloperating range. For example, in visible imaging applications, thesurfaces of lenses 222 may have anti-reflection coatings for visiblelight. When test beam 244 is at a sufficiently long infrared wavelength,it may be strongly reflected by these coatings, thus enhancing theintensity of reflected beams 248 and 250 and making it easier forprocessor 234 to detect them. The anti-reflection coatings on the lensesmay be optimized for high reflection at the emitter wavelength. Asanother example, in depth mapping applications using structured light ofa given wavelength, some of the surfaces of lenses 222 may have abandpass coating, which passes the given wavelength while reflectingradiation outside the passband, including test beam 244.

Alternatively, even when the wavelength of test beam 244 is inside thespectral operating range of the module 240, the high incidence angle ofthe test beam on the lens surfaces will cause the test beam to bestrongly reflected. The anti-reflection coatings on lenses 222 can alsobe optimized to reflect high-angle rays.

Furthermore, if a particular surface is of more interest than others,its coating can be designed accordingly to reflect the wavelength ofemitter 242 with larger efficiency than other surfaces.

In general, however, the methods described herein can be used evenwithout any modification of the coating designs, since a certain amountof radiation is always reflected from the optical surfaces, and theoptical power of emitter 242 and/or the integration time of image sensor226 can be adjusted accordingly.

FIG. 15 is a schematic sectional view of an optical imaging module 260,in accordance with a further embodiment of the invention. Module 260 issimilar to module 240, as described above, except that in the presentembodiment, a pattern 262 is formed on lens D in the area on which testbeam 244 is incident. Pattern 262 causes the reflected test beam to forma predefined geometrical figure on the image sensor, such as acorresponding pattern of focused, reflected beams 264. Processor 234detects movement or changes in the pattern of reflected beams 264, basedon the corresponding electronic image signals output by image sensor226, and is thus able to detect changes in the alignment of lens Drelative to image sensor. Other refractive surfaces in module 260 mayhave similar sorts of patterns for this purpose. Each such surface mayhave a distinctive pattern to enable their respective reflected beams tobe more easily distinguished from one another.

Various sorts of patterns 262 may be used for the purposes of thepresent embodiment. For example, pattern 262 may comprise one or moreradiation-absorbing markings on the lens surface. The markings can bemade thin enough to minimize their effect on the normal operation ofmodule 260. A pattern comprising a few lines of this sort with anappropriate equal spacing between them will give rise to an interferencepattern on image sensor 226. Fourier analysis can be used to extract theextent of relative movement between lens D and image sensor 226 fromchanges in the interference pattern. Additionally or alternatively,pattern 262 may be reflecting, which induces a very strong patternintensity on image sensor 226 and thus makes it possible to decrease theexposure time of the image sensor (and hence reduce its sensitivity toradiation from the scene).

FIG. 16 is a schematic sectional view of an optical imaging module 270,in accordance with yet another embodiment of the invention. In thisembodiment, the emitter comprises a radiation source 272, whichgenerates a test beam 276, and a lens 274, which collimates and directsthe test beam toward one or more reflective surfaces in module 270. Thepictured example shows beams 78 and 80 reflected from the lower, concavesurfaces of lenses D and B, respectively (while omitting the beams thatare reflected from other surfaces for the sake of simplicity). Lens 274may advantageously comprise a microlens, which is decentered relative toradiation source 272 in order to create the desired propagation anglewithin module 270. Although lens 274 is shown in FIG. 16 as a separatecomponent from radiation source 272, the lens may advantageously be amicro-prism-lens, which is integrated with a suitable radiation source,for example as shown in any of FIGS. 1-8 and described above.

Beams 278 and 280 will form respective spots (of different sizes) onimage sensor 226, rather than lines or other large-scale features as inthe preceding embodiments. Collimation by lens 274 thus increases thesignal/noise ratio and sharpness of the pattern, as well as separatingthe reflections from the different surfaces on the image sensor.

Although the figures above show a specific sort of module design, theprinciples of the present invention are similarly applicable to othersorts of imaging modules, with different geometries and arrangements oflenses. All such alternative embodiments are considered to be within thescope of the present invention.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

1. Imaging apparatus, comprising: a housing; imaging optics mounted inthe housing and configured to form an optical image, at a focal planewithin the housing, of an object outside the housing; an image sensor,comprising a matrix of detector elements, which is positioned at thefocal plane in alignment with the imaging optics and which is configuredto output an electronic image signal in response to optical radiationthat is incident on the detector elements; at least one emitter, whichis fixed within the housing and is configured to emit a test beam towardone or more reflective surfaces within the housing, which reflect thetest beam toward the image sensor; and a processor, which is configuredto process the electronic image signal output by the image sensor inresponse to the reflected test beam so as to detect a change in thealignment of the image sensor with the imaging optics.
 2. The apparatusaccording to claim 1, wherein the processor is configured to initiate acorrective action upon detecting that a magnitude of the change isgreater than a predefined limit.
 3. The apparatus according to claim 2,wherein the detected change comprises a shift of the optical image onthe image sensor, and wherein the corrective action comprises processingthe electronic image signal so as to compensate for the shift.
 4. Theapparatus according to claim 1, wherein the at least one emitter isfixed at a location adjacent to the image sensor.
 5. The apparatusaccording to claim 1, wherein at least one of the one or more reflectivesurfaces is an interior surface of the housing.
 6. The apparatusaccording to claim 5, wherein the interior surface is configured as anelliptical mirror, which focuses the reflected test beam to form apredefined geometrical figure on the image sensor, wherein the processoris configured to detect the change in the alignment responsively tomovement of geometrical figure on the image sensor.
 7. The apparatusaccording to claim 1, wherein the imaging optics comprise one or morelenses having refractive surfaces, and wherein the one or morereflective surfaces comprise one or more of the refractive surfaces ofat least one of the lenses.
 8. The apparatus according to claim 7,wherein the imaging optics are configured to form the optical image ofthe object within a predefined spectral range, and the test beam isemitted at a wavelength outside the predefined spectral range, andwherein at least one of the refractive surfaces comprises a coatingconfigured to pass the optical radiation within the predefined spectralrange while reflecting the test beam.
 9. The apparatus according toclaim 7, and comprising a pattern formed on the at least one of thelenses in an area on which the test beam is incident, wherein thepattern causes the reflected test beam to form a predefined geometricalfigure on the image sensor, and wherein the processor is configured todetect the change in the alignment responsively to changes in thegeometrical figure on the image sensor.
 10. The apparatus according toclaim 1, wherein the at least one emitter comprises a radiation source,which is configured to generate the test beam, and a lens, which isconfigured to collimate and direct the test beam toward the one or morereflective surfaces.
 11. The apparatus according to claim 10, whereinthe lens comprises a microlens, which is decentered relative to theradiation source in order to direct the test beam at a desiredpropagation angle toward the one or more reflective surfaces.
 12. Theapparatus according to claim 11, wherein the microlens comprises amicro-prism-lens.
 13. The apparatus according to claim 1, wherein the atleast one emitter comprises a plurality of emitters disposed atdifferent, respective locations within the housing.
 14. A method forimaging, comprising: mounting imaging optics in a housing so as to forman optical image of an object outside the housing on an image sensor,comprising a matrix of detector elements, in alignment with the imagingoptics at a focal plane of the imaging optics within the housing; fixingat least one emitter within the housing so as to emit a test beam towardone or more reflective surfaces within the housing, which reflect thetest beam toward the image sensor; and processing an electronic imagesignal that is output by the image sensor in response to the reflectedtest beam so as to detect a change in the alignment of the image sensorwith the imaging optics.
 15. The method according to claim 14, andcomprising initiating a corrective action upon detecting that amagnitude of the change is greater than a predefined limit.
 16. Themethod according to claim 15, wherein the detected change comprises ashift of the optical image on the image sensor, and wherein thecorrective action comprises processing the electronic image signal so asto compensate for the shift.
 17. The method according to claim 14,wherein the at least one emitter is fixed at a location adjacent to theimage sensor.
 18. The method according to claim 14, wherein the at leastone emitter comprises a radiation source, which is configured togenerate the test beam, and a lens, which is configured to collimate anddirect the test beam toward the one or more reflective surfaces.
 19. Themethod according to claim 18, wherein the lens comprises a microlens,which is decentered relative to the radiation source in order to directthe test beam at a desired propagation angle toward the one or morereflective surfaces.
 20. The method according to claim 19, wherein themicrolens comprises a micro-prism-lens.